Metal carbides, such as tungsten carbide (WC), are an important group of materials that are used in many commercial applications. For some applications, tungsten carbide should have a low level of free carbon and a uniform particle size. Tungsten carbide is useful for making articles which require substantial mechanical strength, such as, for example, dies, cutting tools, and drilling tools.
Synthesis methods for WC may generally be divided into two categories, carbothermal reaction and direct carburization. A carbothermal reaction was first disclosed in the 1930's to the 1940's, in, for example, U.S. Pat. No. 2,285,837. In this method, incomplete reaction results in a mixture of products such as tungsten (W), ditungsten carbide (W2C), and WC. Factors such as the quality of the raw materials, the reactor parameters, and the reaction parameters led to batch to batch variations in the final carbon content of the WC product.
The second method for formation of tungsten carbide is the direct carburization method as described in U.S. Pat. No. 1,876,175. The reaction is simple and the carbon content can be controlled to the stoichiometric level of 6.13 wt %. The direct carburization method has been the standard tungsten carbide powder production method since the 1950s. A drawback of the direct carburization method, however, is that high temperature reaction (e.g., greater than or equal to about 1400° C.) is required, resulting in grain sizes of greater than about 1.0 μm.
In 1980's to the 1990's, two-step processes were utilized to make WC with finer particle sizes, typically 10 nm to 120 nm. One such method was disclosed in U.S. application Ser. No. 20020009411 to Zucker. In the method, tungsten carbide was synthesized from a tungsten precursor compound by heating the precursor compound to a first temperature of at least about 450° C. in a reducing gas composition to form an intermediate tungsten product, and carburizing the intermediate tungsten product in the reactor by heating to a second temperature of at least about 750° C. under a carburizing gas composition comprising at least a first hydrocarbon species to form a tungsten carbide product comprising at least about 98 percent by weight WC. Similar methods were disclosed in U.S. Pat. Nos. 5,372,797 and 5,370,854, and CN 97 1 06622.1.
Another method for making tungsten carbide was described in U.S. Pat. No. 5,567,662. The first step in the method was a partial carburization process to form a mixture of WC, W2C, and W at a low temperature of 1000° C. to 1120° C.; while the second step was adding carbon black to the mixture of W2C. and W and converting to WC at a high temperature of 1200° C. to 1300° C. A drawback of this process was that the high temperature reaction of the second step resulted in partial sintering of the WC. In addition, a post grinding process was required to get final product with a particle size of 0.1 μm to 0.2 μm. Similar methods were disclosed in U.S. Pat. Nos. 5,380,688; 5,166,103; 4,664,899; and 4,008,090; and in the article, “The Direct Production of WC from WO3 by Using Two Rotary Carburizing Furnaces”, Journal of Japan Society of Powder and Powder Metallurgy, Volume 26, No. 3, pages 90+, by M. Miyake, et al.
The two-step carbothermal reaction has an advantage in the precise control of the WC composition. However, many of the previous two-step processes had a first step of partial carburization to form a mixture of W, W2C, and WC at low temperature, followed by a second step of adding carbon to the partially carburized mixture to form WC at high temperature. Disadvantages of these methods include the slow carburization process, high temperature reaction which leads to grain growth, and safety issues resulting from the use of tungsten powder.
Accordingly, there is a need for improved methods of making tungsten carbide, particularly superfine tungsten carbide.